1. Field of the Invention
This invention relates to lubricants for the molding of polyvinyl chloride and copolymers predominantly containing vinyl chloride.
2. Statement of the Related Art
In addition to stabilizers and other processing aids, lubricants are also required for the molding of polyvinyl chloride and copolymers predominantly containing vinyl chloride. Triglycerides containing 12-hydroxy stearic acid residues in the form of hardened castor oil have long been used as lubricants in the processing of rigid PVC. Molded articles which have been produced using hardened castor oil are distinguished by high surface gloss, considerable surface smoothness and minimal white fracture in the glass-clear, transparent and pigmented material. In addition, hardened castor oil can be used in relatively high concentrations without any clouding or exudation occurring (S. Riethmayer, "Gummi-Asbest-Kunststoffe" Vol. 18, 1965, page 428). By virtue of the above-mentioned properties, hardened castor oil is a sought-after lubricant for the processing of rigid PVC, above all in the manufacture of transparent to glass-clear containers for the packaging of foods and beverages.
However, the availability of natural castor oil and, therefore, the availability of hardened castor oil is subject to considerable fluctuations. Poor harvests in Brazil and India where Ricinus communis is principally cultivated lead to a shortage of the starting material for more or less long intervals. Accordingly, there is a need for a substitute for hardened castor oil which, when used as a lubricant for polyvinyl chloride and copolymers predominantly containing vinyl chloride, gives results at least as good as those obtained with the product to be replaced. Above all, the substitute should be obtainable from a more widely available, less crisis-prone raw material.